and many
fleshy roots of vegetables, such as carrots, parsnips, etc., contain
substances known as _pectins_. These are readily soluble in water, and when
dissolved in concentrated solutions in hot water, they set into "jellies"
when the solution is cooled. These jellies carry with them the soluble
sugars and flavors which are present in the fruits, and constitute a
familiar article of diet.

There are undoubtedly several different modifications of the pectins, to
which the names "meta-pectin," "para-pectin," "pectic acid," "meta-pectic
acid," and "para-pectic acid," have been applied. These all seem to be
products of hydrolysis of a mother substance known as "pectose," which
constitutes the middle lamella of unripe fruit, etc. As the fruit ripens,
the pectose is hydrolyzed into the various semi-acid, or acid, bodies
mentioned above. The intermediate products of the hydrolysis are the
pectins, which swell up in water and readily form jellies; while the final
meta-pectic acid is easily soluble in water and resembles the true gums in
its properties. When the middle lamella reaches the pectic acid stage, the
fruit becomes soft and "mushy" in texture.

The pectins more nearly approach to the composition, properties, and
functions of the celluloses than do any of the other groups of organic
compounds. They have been extensively studied in connection with the
parasitism of certain fungous diseases which cause the soft rots of fruits
and vegetables. These parasites usually penetrate the tissues of the host
plant by dissolving out the middle lamella material, which may sometimes
serve as food material for the fungus; but more often the parasite secures
its food supply from the protoplasm of the cell contents. In such cases,
the parasite secretes both a pectose-dissolving enzyme, known as "pectase"
and a "cellulase" which attacks the cell-wall material in order to provide
for the entry of the fungus into the cells. Other enzymes, known as
"pectinases," which coagulate the soluble pectins or pectic acids into
insoluble jellies in the tissues of the plants seem to aid the plant in
resisting the penetration by the parasite.

CELLULOSES

Used in its general sense, this term includes all those substances which
are elaborated by protoplasm to constitute the cell-wall material.
Cellulose proper is a definite chemical compound, whose properties are well
established. In plants, however, this true cell